ture registers are not updated. The previous data is not

changed and remains available.

SMBus Digital Interface

From a software perspective, the MAX6680/MAX6681

appear as a series of 8-bit registers that contain tem-

perature data, alarm threshold values, and control bits.

A standard SMBus-compatible 2-wire serial interface is

used to read temperature data and write control bits

and alarm threshold data. The device responds to the

same SMBus slave address for access to all functions.

The MAX6680/MAX6681 employ four standard SMBus

protocols: Write Byte, Read Byte, Send Byte, and

Receive Byte (Figure 3). The shorter Receive Byte pro-

tocol allows quicker transfers, provided that the correct

data register was previously selected by a Read Byte

instruction. Use caution with the shorter protocols in

multimaster systems, since a second master could

overwrite the command byte without informing the first

master.

When the conversion rate is 8Hz, temperature data can

be read from the Read Internal Temperature (00h) and

Read External Temperature (01h) registers. The tem-

perature data format in these registers is 7 bits + sign

in two’s-complement form for each channel, with the

LSB representing 1°C (Table 2). The MSB is transmitted

first. Extended range extends the temperature data

range of the local and remote sensor to -64°C. Extended

range is activated by setting bit 3 of the Configuration

register to 1.

When the conversion rate is 4Hz or less, temperature

data can be read from the Read Internal Temperature

(00h) and Read External Temperature (01h) registers,

the same as for faster conversion rates. An additional 3

bits can be read from the Read External Extended

Temperature (10h), which extends the remote tempera-

ture data to 10 bits + sign and the resolution to 0.125°C

per LSB (Table 3).

When a conversion is complete, the Main register and

the Extended register are updated almost simultane-

ously. Ensure that no conversions are completed

between reading the Main and Extended registers so

that when data that is read by both registers contain

the result of the same conversion.

±1°C Fail-Safe Remote/Local Temperature

Sensors with SMBus Interface

_______________________________________________________________________________________

9

ACK

7 bits

ADDRESS

ACK

WR

8 bits

DATA

ACK

1

P

8 bits

S

COMMAND

Write Byte Format

Read Byte Format

Send Byte Format

Receive Byte Format

Slave Address: equiva-

lent to chip-select line of

a 3-wire interface

Command Byte: selects which

register you are writing to

Data Byte: data goes into the register

set by the command byte (to set

thresholds, configuration masks, and

sampling rate)

ACK

7 bits

ADDRESS

ACK

WR

S

ACK

8 bits

DATA

7 bits

ADDRESS

RD

8 bits

///

P

COMMAND

Slave Address: equiva-

lent to chip-select line

Command Byte: selects

which register you are

reading from

Slave Address: repeated

due to change in data-

flow direction

Data Byte: reads from

the register set by the

command byte

ACK

7 bits

ADDRESS

WR

8 bits

COMMAND

ACK

P

ACK

7 bits

ADDRESS

RD

8 bits

DATA

///

P

S

Command Byte: sends com-

mand with no data, usually

used for one-shot command

Data Byte: reads data from

the register commanded

by the last Read Byte or

Write Byte transmission;

also used for SMBus Alert

Response return address

S = Start condition

Shaded = Slave transmission

P = Stop condition

/// = Not acknowledged

Figure 3. SMBus Protocols